Temperature measurement is normally performed by measuring the change in some quantity proportional to temperature. Most often in industrial environments, this is performed using a thermocouple. A thermocouple will use the voltage difference between two dissimilar metals and map that voltage to a temperature using a calibration curve. Other measurement techniques may use spectral emissivity, while other temperature measurement devices, such as the standard thermometer, will measure the expansion of a liquid or metal as the temperature changes.
Gas turbine engines provide some of the hottest environments for temperature measurement. Gas path temperatures can exceed 2000° F., which is beyond the melting points of most metals. U.S. Pat. No. 6,489,917 to Geisheimer et al. discloses microwave techniques for measuring other physical parameters within a gas turbine engine, such as blade tip clearance.
Antennas that operate within an extreme temperature environment, such as a gas turbine engine environment, normally use some type of active cooling. Nearly all of the components in the turbine area of the engine contain some type of active cooling since the gas path temperature is normally hotter than the melting point of the metals. The cooling air is normally provided by air taken out of the compressor and ducted through the outside of the turbine case as well as through the blades themselves. Therefore, a ready source of cooling air is available, and the antenna installation is typically designed to use some of the cooling air in order to keep the antenna at temperature that will allow it to function for many thousands of hours.
The cooling air design is performed by creating a thermal model or performing thermal calculations that make assumptions about the gas path temperature, the cooling air temperature and pressure, as well as the rate at which heat is transferred from the gas path to the turbine wall. Typical design values represent worse case scenarios and therefore, it can be difficult to accurately determine the operating temperature of the probe.
Embedding thermocouples or other temperature measurement devices within the antenna is possible, however, it represents additional complexity. Often, space is at a premium and routing out another wire as well as dealing with reliability issues of operating at a thermoucouple at such high temperature is undesirable. Therefore, it is desirable to derive a technique to measure temperature using the existing antenna structure.
Antennas are used to transmit and receive electromagnetic energy. Typically, they are used within ambient temperature environments and are used in such devices as mobile phones, radios, global positioning receivers, and radar systems. One particular type of antenna, known as a microstrip or patch antenna, is made by forming a geometric pattern of metal on a dielectric substrate. Many such designs are constructed with printed circuit board etching processes common in circuit board manufacture. The geometry of the design is typically rectangular or circular, but other geometries are possible to provide enhanced performance such as increased bandwidth or directionality.
The materials used as the dielectric substrate for an antenna are often circuit board materials, such as FR4, or other ceramic materials such as alumina (Coors AD-995 or similar). All dielectric materials exhibit some change in dielectric constant as a function of temperature. The center frequency of a patch antenna is based on a designed resonance frequency that is a function of the metallization geometry as well as the dielectric constant. For most dielectric materials, the change in dielectric constant is non-linear as a function of temperature, with the rate of change of the dielectric constant increasing as the temperature increases.
The center frequency of the antenna can be measured using a network analyzer, or other similar measurement device, that measures the amount of energy transmitted by the antenna. The typical measurement method measures the antenna reflection coefficient, which consists of transmitting a signal down a cable and measuring the amount of energy that is returned. If the energy is not returned, then it is assumed that most of it passed out through the antenna and is not returned. The reflection coefficient is measured across a range of frequencies to determine the frequency where the antenna most efficiently transmits.
For most antenna applications, the temperature of operation is low enough that the change in dielectric constant temperature does not significantly impact how the antenna functions. However, in applications such as measuring tip clearance inside gas turbine engines, the temperature change can be high enough that significant changes in the dielectric constant can be seen.
In view of the foregoing, it will be appreciated that that the act of measuring the temperature level faced by an antenna located within a high temperature environment requires a different approach than that found in the prior art. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.